The goal of these studies is to investigate mechanisms involved in the initiation of division of cultured animal cells. We have found that division of some kinds of cells is stimulated by added glucocorticoids, and that other kinds of cells are unresponsive to glucocorticoids. However, the responsiveness of the cells appears not to be determined by the presence or absence of glucocorticoid-specific cell components like the cytosol receptor. Our experiments have shown that serum is required for the initiation of cell division by glucocorticoids, but that treatment of cells in serum-free medium with glucocortidoids can elicit some of the activities which appear to be involved in the initiation of cell division. We are now studying these activities and their involvement in the initiation. We are also continuing our studies on the mechanism by which trypsin and thrombin initiate division of cultured chick cells. Our recent studies have shown that trypsin can initiate without entering the cells. We are now looking for the cell surface changes that are involved in the initiation. This will involve an examination of cell surface proteins before and after trypsin treatment in cells that are responsive and cells that are unresponsive to the initiatton by trypsin.